This invention relates generally to a motor control device of an electric sewing machine motor and more particularly to a control device of this type which, in the process of stopping the sewing machine, can accurately stop the sewing needle at a predetermined stagnant position.
Referring first to FIG. 9, there is shown a structure of conventional sewing machine which comprises of a sewing head portion 1, a detector unit 2 for detecting the rotational speed of sewing machine and the position of the needle, a driving motor 3, a control unit 4, and a control pedal 5, wherein the power supply of the sewing machine is shown in FIG. 10. A flying wheel 3A within the motor 3 is driven in a constant revolution. When the coil of the electromagnetic clutch 3B is supplied with exciting current of various strength, through frictional coupling or eddy-current coupling mechanism, the torque of the flying wheel 3A is proportionally transmitted to the driven shaft 3C. The stopping action of the sewing machine is performed by the friction brake 3D. When the sewing machine is stopped, the rotation of the electrical motor is still maintained. As a result, great amount of electrical energy is wasted. On the other hand, the inductance of the coil of the clutch 3B is large, and there will be time delay in response at the instanct when the current through the coil is cut off or turned on. Moreover, with the mechanical reciprocating operation of the clutch, the response time of the whole system is further increased. Therefore, this will result in an unstable speed control, poor response and inexact stagnation position of the needle.
The durability of the friction clutch 3B and the friction brake 3D is increased by using friction disc made of soft wood coated with anti-wear material. However, the sewing machine usually operates at a moderate speed, i.e., there is difference in rotational speed between the flying wheel 3A and the clutch 3B, as a result, the wearing of soft wood will increase the gap between the contact surface and the soft wood (the gap distance should be kept within 0.3-0.5 mm). This effect results in the increase of the response time, and the control system will be more unstable. Particularly, for the case of the friction brake, both the delay of exciting current and the increase of gap distance will retard the braking process, and then increase and change the braking distance of the needle. These effects will degrade the precision of the positioned stagnation of the needle.
If the eddy-current clutch is employed, contact between transmission shafts can be avoided and the durability is thus increased, however, due to the low efficiency, the exciting current must be increased which may result in overheat or significant deformation in some parts of the clutch. The driving motor not only comprises a main body, but also includes a flying wheel, a friction or eddy-current clutch, a friction brake, thus the machine is rather awkward, and due to the co-axial rotation, it is necessary that each gap be accurately adjusted, thus the cost and quality of the product are affected.
In order to mitigate the above-mentioned drawbacks, it is proposed that conventional DC permanent magnetic (PM) motor or brushless motor is employed. However, the frequent on and off operation of the sewing machine will damage the brush of the DC PM motor, which thus increases the cost of operation. When employing the brushless motor in sewing machine, an electronic motor control device must be employed to achieve a stable speed control and stop the needle at a present stagnation.
The speed control of a DC motor is performed by controlling the mean current flow through the stator winding of the motor. The greater the mean current is, the higher the speed is. If the speed is to be reduced, current with reverse direction is applied, and the greater the reverse current is the greater the retarding torque is.
In order to stagnate the needle at a specific preset position, the control device is provided with a needle-positioning signal detector which will send out a needle positioning signal at the moment when the needle reaches a specific preset position, and at this moment the control device starts the braking process and supplies a reverse braking current until the needle is completely stopped. In the conventional design, one of the key points is to determine the exact end point of the braking process and still keep the braking distance can be pre-compensated and precisely adjusted. In the prior art, different control mechanisms of the braking process have been suggested, but an accurate positioned stagnation of the needle still cannot be achieved. Some of factors making this trouble are the drift of the DC (direct current) level of the speed detector due to the variation of the ambient temperature, and the dependance of the braking distance on the instantaneous speed of the motor at the moment when the braking process just begins, which is varied with the working condition of the sewing machine (the low speed range is within 120-240 RPM) and generally not a constant.